The original tale of The Little Mermaid had the Witch as not being evil, just neutral and clearly warning of the risks. In at least some versions, she even informs the mermaid's sisters about how to save her.
While many folklorical black dogs are malevolent, some are this trope instead. Eg. the Gurt Dog of Somerset in Britain is thought to protect children and lone travellers.
According to Alpine folklore, the demonic goat figure Krampus is the polar opposite of Santa Claus in that while Santa rewards good children with gifts, Krampus punishes naughty children using harsh punishments. It's because of this that he's often depicted as a good guy within countries that honor Krampus-related traditions in spite of him being a goat demon (although you might not get that within certain works of fiction).